Traditional equipment and methods for injection die casting of molten metal into molds are well known. Such metals would include aluminum, steel, wrought irons, brass, bronze and various exotic alloys, among others. In injection die casting machines, a metal shot sleeve is securely fitted to the mold platen in fluid communication with the mold cavity, as outlined more fully below. The shot sleeve extends outwardly from the mold platen and is adapted to receive the molten metal through an ingress port therein, which port is adapted to pass the molten aluminum into the mold cavity. Shot sleeves are typically anywhere from 24" to 48" in length and from 6" to 14" in diameter, and are typically made of a high-grade steel, such as H-13 grade steel, which steel is expensive. Further, the heat treating process used to harden steel requires high temperatures, which often causes the steel to warp. The shot sleeve has a cylinder bore extending the length thereof, which cylinder bore is typically circular in cross section, and is in fluid communication with the ingress port at one end and the mold cavity at the opposite other end. Further the cylinder bore of the shot sleeve must be machined to within tolerances of about 0.001" to about 0.002" in order to receive a cooperating piston in sliding relation therein, which machining is an expensive and time consuming operation. Resultingly, a typical shot sleeve may cost in the order of about $750.00 to about $4,000.00.
In use, a first end of the shot sleeve is entered into a mounting hole in the mold platen, to which platen it is securely fastened. The first end of the cylinder bore thus extends through the mold platen and connects through its open end to the cavity of the mold, which mold is securely mounted on the opposite side of the mold platen. The cylinder bore of the shot sleeve is in this manner mounted in the injection die casting machine in fluid communication with the mold cavity. Molten metal, which is typically about 1450.degree. F., is then poured either by a robotically controlled ladle or a hand operated ladle into the cylinder bore of the shot sleeve through the ingress port. The molten metal starts to cool from its initial temperature of about 1450.degree. F. as soon as it is introduced into the cylinder bore of the shot sleeve. It is important that the molten metal reach the mold while it is still fully molten in order to ensure the ultimate quality of the metal casting. A large amount of heat is transferred from the molten metal into the shot sleeve, largely because the shot sleeve is made of high-grade steel, which is a highly heat-conductive material. Such high heat loss is undesirable, since the amount of heat energy lost to the shot sleeve must be added to the original molten metal in the form of a higher initial temperature. Raising the initial temperature of the molten metal unnecessarily, especially over 1000.degree. F., is undesirable since it is very expensive to heat metal, especially molten metal, past a temperature of about 1000.degree. F. Indeed, the melting point of aluminum, for example, is in the order of 1200.degree. C. Typical initial temperatures of molten aluminum, when prior art steel shot sleeves are used, are in the order of 1450.degree. F. The extra heat energy above the melting temperatures is merely required to keep the metal in its molten state until it is in place in the mold cavity.
Due to cost considerations, it is typically required that the shot sleeves be able to withstand 40,000 shots of molten metal without failure or excessive wear. Prior art shot sleeves typically have a useful life expectancy of about 10,000 to about 15,000 shots maximum, since they are subjected to various harsh environmental conditions, such as exposure to corrosive materials at an extremely high temperature and pressure. This is mainly due to the fact that molten metal is very corrosive, because of additives such as silicon. Aluminum, as used for casting automotive parts, for example, has an especially high silicon content, which silicon causes the inner wall of the shot sleeve, which defines the cylinder bore, to eventually become corroded and, therefore, unusable. Further, when the molten metal is poured into the hollow core of the shot sleeve, the shot sleeve is subjected to very high thermal shock, which can eventually cause alterations to the material properties of the high-grade steel of the shot sleeve.
During the actual casting operation, there is a reciprocating piston situated in sliding relation within the cylinder bore of the shot sleeve. Before the molten metal is poured into the shot sleeve, the piston is retracted to the second end of the shot sleeve, which is the end opposite to the mold cavity. After the shot sleeve has been filled with molten metal, the piston is advanced along the cylinder bore of the shot sleeve in order to push the molten metal into the mold cavity under pressure. During such advancement of the piston, molten metal pressures in the order of about 6,000 PSI to about 14,000 PSI, are encountered in the shot sleeve. In order to: retain the molten metal within the cylinder bore of the shot sleeve; preclude the molten metal from escaping past the piston; and, retain the high pressure within the shot sleeve and mold, the piston is adapted to functionally seal against the inner wall defining the cylinder bore of the metal shot sleeve. Thus, the inner wall must be machined to within tolerances of about 0.001" to about 0.002", and must remain within a very small amount of its original shape and size throughout its useful life. Since a conventional steel shot sleeve does not remain within very close tolerances for more than about 10,000 to about 15,000 shots, which is significantly shorter than the life expectancy of a mold, the shot sleeve would need to be replaced at least once during the life of the mold. Such replacement is undesirable since the down time of the die casting machine during replacement and the labour to replace a shot sleeve are very costly. Typically, a mold must be cooled considerably from its operating temperature of between about 300.degree. and about 400.degree., which can take several hours, so as to be safe to work on. The mold must be heated up to operating temperature. It is not uncommon for the "down time" required to change a shot sleeve and all of the necessary related other parts to be in the order of 8 hours. Such down time and labour is considerable, having regard to the amount of disassembly required, and further including the removal of coolant lines. Further, prior art shot sleeves are costly, as mentioned previously, up to about $4000.00.
The sealing of the piston against the inner wall defining the cylinder bore creates relatively high friction therebetween, which necessitates that a suitable lubricant, typically a graphite based release agent, be used on the piston. Due to the high temperatures of the molten metal, the lubricant is to a large degree burned off, which causes undesirable acrid smoke and fumes, which are considered a hazard from a worker health and safety standpoint. The inclusion of excess lubricant within the molten shot may also adversely impact upon the quality of the metal part being cast, due to impregnation of lubricant into the metal being cast. Further, any excess lubricant must eventually be disposed of, which is considered to be an environmental problem.
At the end of an injection molding cycle, it is common to have a portion of the molten metal solidify in the cylinder bore of the shot sleeve, adjacent the piston. This solidified portion is commonly referred to in the industry as a "biscuit". A biscuit is removed after each cycle by further advancement of the piston. The biscuit is usually reasonably tightly lodged in the first end of the cylinder bore of the shot sleeve, and resultingly there is a great deal of friction between the biscuit and the shot sleeve when the biscuit is pushed out by the piston. Resultingly, the first end portion of the inner wall of the cylinder bore of the shot sleeve (i.e. the position closest to the mold) is subjected to a great deal of wear during removal of the biscuit.
A further problem encountered in the use of prior art shot sleeves arises from the fact that it is often desirable to include radially extending fluid passageways, called "runners" in the first end face of the shot sleeve, where it fluidly connects as aforesaid with the mold cavity. Such runners are included to allow for increased flow of molten metal from the cylinder bore of the shot sleeve into the mold cavity, or into certain portions of the mold cavity. The use of runners is advantageous because it allows a mold to be filled more quickly and more evenly, and further allows the mold cavity to be more deeply recessed into the cover side of the mold, thus giving more latitude to the setting of a particular part into the mold. It is difficult and expensive to machine such runners into the first end face of the shot sleeve because of the large length and high mass of conventional shot sleeves, which makes handling difficult. Runners, although desirable, are frequently omitted from prior art shot sleeves because of the difficulty and expense to machine them. Certain types of parts are difficult to injection die cast without the inclusion of runners. It is also known that some types of exotic alloys are difficult to injection die cast without the inclusion of runners in the shot sleeve, which makes the die casting of these alloys extremely expensive. Further, in the event that the mold is to be changed before the shot sleeve is worn out, the shot sleeve must be changed anyway in order to allow for the inclusion of appropriate runners in the new shot sleeve. The shot sleeve must then be discarded, even though it is still functional.
One type of prior art shot sleeve that is directed towards solving some of the problems of earlier types of prior art shot sleeves is known as a "split sleeve" type shot sleeve. A split sleeve type shot sleeve is basically a two-piece shot sleeve radially bisected at the midpoint of its length. The two sections of the split sleeve are pinned together to form a fully functional shot sleeve. One section is attached to the mold platen and the other section extends outwardly from the mold platen and is removable from the retained section by removing pins or bolts that secure the two sections together. It is thereby possible to replace only the one section of the split sleeve furthest from the mold, often called the "back end" of the split sleeve, without replacing the other section of the split sleeve. This is desirable because the "back end" of the split sleeve, which contains the ingress port, is subjected to greater thermal shock and corrosion than is the other section of the split sleeve, and therefore typically becomes worn out earlier than the "front end" connected to the mold platen. Further, it is not necessary to disassemble the "front end" of the shot sleeve from the mold to replace the entire split sleeve, if only the "back end" needs replacing. A split sleeve type shot sleeve does, however, retain the aforementioned problems concerning heat transfer wear, and lubrication. Moreover, new problems are introduced with the use of split sleeve type shot sleeves. That is, the two sections of the split sleeve may not be entirely concentric with one respect to another within the 0.001" to 0.002" previously discussed. Such lack of concentricity causes premature wear where the piston meets the joint between the two halves of the cylinder bore of the shot sleeve. Also, lack of concentricity between the two cylinder bore halves can make it difficult to maintain a sealed relation between the piston and the inner wall of the hollow core of the shot sleeve in both halves of the cylinder.
It is therefore an object of the present invention to provide a shot sleeve for die casting machines that overcomes these and other related problems associated with prior art shot sleeves.
More particularly, it is an object of the present invention to provide a shot sleeve that reduces the amount of heat lost by the molten metal being cast while in the shot sleeve.
It is another object of the present invention to provide a shot sleeve that is more resistive to the corrosive action of molten metals than prior art shot sleeves.
It is a further object of the present invention to provide a shot sleeve that is more resistive to physical wear than prior art shot sleeves.
It is another object of the present invention to provide a shot sleeve that is more resistive to high thermal shock than prior art shot sleeves.
It is still a further object of the present invention to provide a multi-part shot sleeve that is easier and less expensive to maintain in standard operating conditions than prior art shot sleeves.
It is an object of the present invention to provide a shot sleeve that substantially reduces the need for using supplemental lubricants in the die casting process.
It is an object of a preferred embodiment of the present invention to provide a shot sleeve that comprises a removable and replaceable end collar means that is adapted to permit runners to be readily and easily machined therein for enhanced introduction of molten metal into a mold cavity, thereby allowing for increased design flexibility of molds.
It is another object of a preferred embodiment of the present invention to provide a shot sleeve that comprises a removable and replaceable end collar means that can be replaced independently of the need to replace the remaining components of the shot sleeve.